Method and compositions for double coating a traveling web



United States Patent 3,362,845 METHOD AND COMPOSITIONS FOR DOUBLECOATING A TRAVELING WEB Maurice M. Brundige, Laurel, Md assignor to WestVirginia Pulp and Paper Company, New York, N.Y., a carporation ofDelaware No Drawing. Filed June 22, 1967, Ser. No. 647,936 12 Claims.(Cl. 117--76) ABSTRACT OF THE DISCLOSURE Process of double coating atraveling web wherein coarse pigment materials having an averageparticle size of at least about 1.6 microns are employed in the base ortop coats to reduce and prevent streaking when the top coat is appliedby a blade or knife coater.

Cross-reference to related application- This is a continuation-in-partof U.S. patent application Ser. 332,292, filed Dec. 20, 1963, and nowabandoned.

Brief summary 0 the invention This invention relates to the coating ofweb material. In general, the coating of web material, such as paper,paperboard, and the like, may be performed by the use of roll coaters,air knife coaters, or blade coaters. Each type of coater has advantagesand disadvantages peculiar to its own type. For example, blade coatershave a distinct advantage of providing a very smooth surface even at lowcoat weights because of a leveling or filling action which isaccomplished by the blade. Blade coaters find great utility, therefore,in the coating of paper and paperboard which is subsequently subjectedto a printing operation, since the latter operation is enhanced if themedium to be printed has a smooth surface.

In the coating of paper and paperboard, it is sometimes desirable toapply two layers of coating to a traveling Web in order to obtain acoated surface with maximum smoothness and improved ink holdout. Thisinvention concerns such a double coating process. A first layer ofcoating, comprising an aqueous dispersion of mineral pigment and anadhesive, is applied to a traveling web by one of the above mentionedcoating apparatuses, and the coated web is dried. Then a second layer ofcoating, comprising an aqueous dispersion of mineral pigment and anadhesive, is applied, and the web is again dried. According to thisinvention, the apparatus used to apply the first coating layer, or basecoat, is not important. In the preferred method, the base coat isapplied by a blade coater. Regardless of how the base coat is applied,the second coating layer, or top coat, is applied by a blade coater inorder to produce a final coated sheet of highest quality.

A web of paper or paperboard has an inherent roughness or unevennesswhich is attributable to its formation on a paper machine. A first layerof coating over the paper or paperboard fills in the roughness of theweb to a great extent and tends to seal the fibers in the web. Thesecond layer of coating enhances the filling of the roughness of the weband improves the continuity of the final coated surface, since it isapplied over a relatively smoother surface than is the first layer ofcoating.

With the increased use of blade coaters in the industry to obtainmaximum quality has come the problem of scratching or streaking whichbecomes more pronounced when the top coat is applied by a blade coaterover the previously coated web. The base coat fills in low areas of theweb and reduces the inherent roughness of the web. Oftentimes, when thetop coat is applied by blade coater,

scratches or streaks occur in the top coat as it is being smoothed bythe blade. Such paper is a reject product primarily because ofappearance. The streaks are caused by hard particles in the coatingwhich become lodged in the nip between the blade and the traveling web,and as the web travels under and past the blade, the absence of coatingon the web, caused by the lodged particle, takes the form of a streak orscratch on the web. The streaking continues until either the particlebecomes dislodged and passes under the blade or the particle is abradedaway.

The hard particles which cause the streaking may be dried adhesive ordried coating from the coating supply system, or they may be foreignmaterials which have not been removed from the coating system. It wasbelieved that increased screening of the coating by use of finer screenswould eliminate the problem. It is common in the industry to screencoatings with screens having mesh sizes in the range of to 150. Use ofscreens with mesh sizes above is not commercially feasible, but in anattempt to eliminate the scratching problem, coatings were passedthrough 325 mesh screens with the result that while a slight improvementin scratching was noted, the problem was far from solved.

Through much experimentation, it has been found by the inventor that,surprisingly, the rougher the web is when it is presented for the secondcoating application, the less the amount of scratching that occurs inthe top coat. It has been found that when the web is somewhat smoothedby a first layer of coating, scratch-producing particles in the coatingfor the top coat have a more diflicult time passing under the blade of ablade coater. This situation occurs, it is believed, because there arerelatively few low areas in the web which can accommodate the lodgedparticles and carry them through the nip formed by the web and theblade. When the first layer of coating is applied, a particle whichlodges under the blade soon becomes dislodged, either by finding a lowarea in the web or by being abraded away by the inherent roughness ofthe web until such time as the particle becomes of a size tosuccessfully pass under the blade. But due to the filling of the lowareas of the web by the base coat, the web is in a relatively smoothcondition such that its roughness, and hence its abrasive effect on alodged particle in the top coat, is reduced. A particle in the top coatwhich lodges under the blade is thus not removed by the web and maylodge indefinitely.

It has been found that the advantages of a double coating operation,such as higher coat weights, increased opacity, printing smoothness, anduniformity of ink holdout, can be obtained and yet scratching can bedrastically and significantly reduced by applying a base coat which haswhat may be termed as a built-in, preselected roughness. Contrary tonormal procedure, it has been found that an increase in the roughness ofthe base coat reduces the amount of scratching that occurs when the topcoat is applied by a blade coater. Base coats containing such pigmentsas coarse calcium carbonate, ground marble, and sand as replacements forpart or all of the regular coating pigments have been found to producethe properties desired in the base coat for the reduction or eliminationof scratching in the top coat. This substitution of coarse materials inthe base coat provides a means for controlling the base coat roughnessby selection of the particle size of the coarse materials, whichparticle size may be varied to obtain the roughness necessary toeliminate scratching in the top coat. By building in a roughness in thebase coat, scratch-producing particles in the top coat which becomelodged under the blade during the smoothing of the top coat are carriedaway from the blade by the web or the roughness of the base coat abradesaway the lodged particles until they successfully pass under the blade.

Replacements in the base coat of coarse pigment materials having anaverage particle size of at least about 1.6 microns for from 10 to 100%of the regular coating pig ments, such as regular coating clay orprecipitated chalk, have been found to be effective in the reduction ofscratching when the top coat is applied over the base coat by a bladecoater. The invention is not limited to the use of a particular coarsepigment.

As one skilled in the art of paper coating will readily appreciate, theinvention is not limited to the use of a particular adhesive. Theadhesive or binder used to bind the pigment in the coatings to thetraveling web can be any of the conventional water-dispersible adhesivesused in aqueous paper coatings. Such adhesives are starches, casein, andother proteins such as alpha and soybean protein, polyvinyl alcohol,resin emulsions, resin latices, and combinations of adhesives such asstarch-latex or soya protein-acrylic resin mixtures, all of which may bereferred to as film-formers. The invention is not limited to the use ofa particular starch as any of the usual starches, such as dextrins,oxidized starches, cationic, acid modified, pearl, and ethylatedstarches can be used.

Detailed description The invention will be better understood byreference to the following illustrative examples.

EXAMPLES 1-4 In Examples 1-4, a standard base coating of the followingformulation was employed:

A standard top coating of the following formulation was employed:

Parts Lustra clay (a premium grade clay) 80 Columbia clay 20 RhoplexB15, 18% based on the pigment 18 Protein (soya), 3 /2 based on thepigment 3 /2 In all the data given throughout this specification, unlessotherwise noted, the solids content of the base coatings ranged from52.1 to 53.0%, and the solids content of the top coatings ranged from57.8 to 59.6%, after reduction with water to form aqueous dispersions ofcoating. Coat weights are expressed as pounds per 3300 square feet ofcoated web materials.

Fourteen point board was double coated with the above base and top coatformulations by inverted blade coaters, such coaters being known in theart, as for example the coater described in my US Patent 3,149,005. Afirst coating layer was applied to the traveling web of board in knownmanner, as by an applicator roll; the coating was smoothed with a doctorblade in known manner; the first layer of coating was substantially setby passing the traveling web into a drying zone, as for example a zonecomprised of heated drier drums; a second layer of coating was appliedover the first coating layer; the second layer of coating was smoothedwith a doctor blade; and the resultant double coated web wassubstantially dried. A length of the so-coated board, 100 feet, chosenat random, was examined for scratches which were evaluated as short (3inches and less in length), medium (between 3 inches and 12 inches inlength), and long (more than 12 inches in length). The following datawas gathered:

STANDARD RUNS-TOP COATS SCREENED THROUGH A 100 MESH SCREEN Scratches on100' of Web Example Coat Coat Weight Long Medium Short Total {iiifiiiji31% 1G 12 8 36 @3311: iii 25 25 3 i%if1;::: iii} 24 21 9 STANDARDRUN-TOP COAT SOREENED THROUGH A 200 MESH SCREEN Basevn 2.6

EXAMPLES 5-13 Fourteen point board was doubled coated with the base andtop coat formulations of Examples 1-4, in the same manner as above,except that replacements for the coating clay in the base coat ofExamples 1-4 were made with coarse calcium carbonate, ground limestone,and sand. The data for these runs was as follows:

TOP COATS SCREENED THROUGH A 100 MESH SCREEN Percent Scratches on 100 ofWeb Example Coat (Format Replatlement Material erg av Long Medium ShortTotal Base 2.6 I {Tmnu 3O 4 50 Sand o 0 2 2 g 50 Gamaco-T o o 0 o 4.1 M10 .do 14 1o 2s Ground Marble.-- is e s 24 Base 3.3 9 39 d0 a 5 3 11 TOPCOATS SCREENED THROUGH x 200 MESH SCREEN Base 1.4 10 m 100 "men 0 o o o11 do 0 1 1 2 Basc 3.1 12 i Gamaco-T 0 i 2 a ase. .8 13 L2 100 ..do 0 01 1 Comparing Examples 513 with Examples 1-4, it can be seen from thedata that the amount or scratching in the top coats was greatly reducedby use of the substituted materials for regular coating clay in the basecoats, when compared to the amount of scratching that occurred when thestandard base coat formulation was employed. It can also be seen that asgreater substitutions for clay were made, the level of scratchingdecreased. There is an improvement with only a substitution in the basecoat; the preferred range is from 25 to 100% substitution.

The Gamaco-T is a material marketed by the Georgia Marble Company and isprepared by crushing stone of high calcium carbonate content, to yield aproduct which typically has a calcium carbonate content of 98%. Ninetypercent of the particle size range is finer than 10 microns, with 80%below 6.5 microns, 58% below 3 microns, 40% below 2.2 microns; theaverage particle size is approximately 2.5 microns.

It has been found that Atomite, marketed by Thompson, Weinnman & Co.,which is also a material prepared by grinding stone of high calciumcarbonate content to yield a product which typically has a calciumcarbonate content of 98.2%, can also be used successfully. The particlesize range for Atomite is from 0.5 to 10 microns, with an averageparticle size of approximately 2.5 microns.

The ground marble used is known as Georgia Marble Co. 525 and has thefollowing particle size characteristics: 75% between 5 and 25 microns,with 90% below 25 microns; average particle size of 12 microns.

The sand used above was 15 micron Minusil which is white, crystallinesilica of angular shape, and two dimensional, primarily as slivers orvflakes, marketed by the Pennsylvania Glass Sand Corporation. Thissilica has the following particles size characteristics: 99.7% belowmicrons; 98.2% below 15 microns; 90% below 10 microns; 60% below 5microns; 40% below 3 microns; 28.5% below 2 microns; average particlesize of 2 microns. 10 micron Minusil has also been employed succoat hada solids content of 58%. It can be seen that the scratch level is in thesame range as the standard formulations, and that substitution of thePurecal O for regular coating clay in the base coat was ineffective inreducing scratching in the top coat. Purecal O, a well knownprecipitated calcium carbonate, has an average particle size ofapproximately 0.25 micron.

While the great utility of the ground and coarse materials is not fullyunderstood, it is known that ground calcium carbonates are generallylarger in particle size than precipitated calcium carbonates, and it isbelieved that particle size, along with the abrasive nature of theground materials, is responsible for the improvements in scratchingshown throughout this specification. Thus, it is preferred to use in thebase coat of a double coating process a coarse pigment material whichhas an average particle size of at least 1.6 microns, and the preferredcoarse pigment materials are calcium carbonate and silica, and theirequivalents.

EXAMPLES 15-1 6' It has been found that replacements of at least 20% ofthe coating clay in the top coat with the coarse materials hereindisclosed will also reduce scratching in the top coat. If more than a20% replacement is made in the top coat, printability of the finalcoated web may be adversely affected, but more than a 20% replacementmay be utilized. The following data is illustrative of a 20% replacementof Gamaco-T or 10 micron Minusil for the coating clay in the top coat.The base coat was a standard formulation of 100 parts regular coatingclay (Columbia clay), with 20% binder (16% acrylic resin and 4% protein)based on the clay content; solids content of 52.7%. The top coatformulation was 80 parts premium clay, 20 parts of either Gamaco-T or 10micron Minusil, with 21 /2% binder (18% acrylic resin and 3 /2% protein)based on the clay content; solids content of 58% in both instances. Thetop coats were screened through a 100 mesh screen before application.

TOP COATS SCREENED THROUGH A 100 MESH SCREEN Percent Scratches on 100 ofWeb Example Coat Coat Replacement Material Weight of Clay Long MediumShort Total 15 {Base 4.5 1 20 Gamaco-T O 3 4 7 Top 2.2 J 16 Base 4. 5 2010 Micron 2 4 1 7 {T0p 2. 4 Minusil.

EXAMPLE 14 Precipitated materials used in the base coat rather thanground materials have been found to be ineffective in reducingscratching the top coat, as the following data illustrates:

Coat Percent Re- Scratches on Run Coat Weight placement Material 100 ofWeb,

of Clay Total 1195 .g; 70 Purecal o...

The top coat was screened through a 100 mesh screen. The base coat has asolids content of 57% and the top Comparing the results of Examples 15and 16 with Examples 1, 2, and 3 above where the standard formulationswere used, it is seen that an effective reduction in the amount ofscratching in the top coat was achieved by using the coarse materials inthe top coat. The reason for improvements in scratching when using thecoarse materials in the top coat is not fully understood, but it isbelieved that a lodged particle is abraded away by the coarse particlesin the second coating layer as the coarse particles flow past the lodgedparticle.

EXAMPLE 17 As previously stated, any of the usual paper coatingadhesives can be employed in practicing the methods of this invention.In this instance, the base coating used in Example 6 above, containingas pigment equal parts of clay and Gamaco-T, was employed again, exceptthat the acrylic-resin-p-rotein adhesive was repleced by 17 parts byweight of alpha protein. The base coating was applied at a coat weightof about 4 pounds, and the coated paper was dried. Then about 4 poundsof the standard top coating from Examples 1-4 was applied over the driedbase coat, and the scratch level of the so-coated paper was substantially the same in excellence as that exhibited in Example 6.

Additional runs have shown that the resin-protein adhesives set forth inthe base and top coats of Examples 1-4 can be replaced by otherconventional paper coating adhesives. For example, in one run, Example 6was repeated, except that the resin-protein adhesive in the top coat wasreplaced by 16 parts by Weight of alpha protein. The scratch level wassubstantially the same as in Example 6. Further, when the coarsepigments are used in the top coats, as in Examples 15 and 16, any of theconventional adhesives can be used in the top coat to bind the coarsepigments to the paper. In one run, Example 15 was repeated except thatthe resin-protein adhesive in the top coat was replaced by 18 parts byweight of an ethylated starch, and the resultant scratch level of thefinal coated paper was substantially the same as that of Example 15.

The base stock to he coated in the manner of this invention may besubjected to a conventional surface size treatment, and if so, the sizetreatment is not considered to be one of the layers of coating accordingto this invention. However, the coarse materials employed in thisinvention may be applied at the size press, in which case, the sizing isconsidered to be the base coating.

While ground calcium carbonate and silica have been disclosed asmaterials to be incorporated in the base or top coats of a doublecoating process, other coarse particles, having an average particle sizeof at least about 1.6 microns, may also be used and combined in coatingformulations with the usual coating clays which range in particle sizefrom about 0.4 to 0.8 micron, to reduce scratching or streaking in thetop coat. Therefore, various ingredients may be substituted for thecoarse materials set forth herein without departing from the spirit ofthis invention or the scope of the appended claims.

I claim:

1. In the method of coating a traveling web of paper, paperboard, andthe like which involves the steps of (a) applying to the traveling web afirst layer of coating comprising an aqueous dispersion of mineralpigment having an average particle size up to about 0.8 micro andfilmforming binder, (b) substantially setting the first coating layer,(c) applying over the first coating layer a second layer of coatingcomprising an aqueous dispersion of mineral pigment having an averageparticle size up to about 0.8 micron and film-forming binder, (d)smoothing the second layer of coating with a doctor blade, and (e)substantially drying the resultant coated web, the improvement for thereduction of scratching in the second coating layer which comprisesapplying as the first coating layer a coating wherein the mineralpigment comprises at least about 10% of a coarse pigment material whichhas an average particle size of at least about 1.6 microns.

2. The method of claim 1 in which the coarse pigment material isselected from the group consisting of ground calcium carbonate andsilica.

3. The method of claim 2 wherein the coarse pigment material comprisesfrom about 10 to 100% of the mineral pigment in the first coating layer.

4. The method of claim 2 wherein the coarse pigment material comprisesfrom about 25 to 100% of the mineral pigment in the first coating layer.

5. The method of claim 1 wherein the binders in the coatings employed insteps (a) and (c) are water-dispersible.

6. The method of claim wherein the binders include water-dispersibleresins.

7. In the method of coating a traveling web of paper,

paperboard, and the like which involves steps of (a) applying to thetraveling web a first layer of coating comprising an aqueous dispersionof mineral pigment having an average particle size of up to about 0.8micron and filmfcrming binder, (b) substantially setting the firstcoating layer, (c) applying over the first coating layer a second layerof coating comprising an aqueous dispersion of mineral pigment having anaverage particle size of up to about 0.8 micron and film-forming binder,(d) smoothing the second layer of coating with a doctor blade, and (e)substantially drying the resultant coated web, the improvement for thereduction of scratching in the second coating layer which comprisesbuilding a roughness in the first coating layer applied in step (a)above by replacing at least about 10% of the mineral pigment with acoarse pigment material which has an average particle size of at leastabout 1.6 microns.

8. The method of claim 7 in which the coarse pigment material isselected from the group consisting of ground calcium carbonate andsilica.

9. The method of claim 7 wherein the binders in the coatings employed insteps (a) and (c) are Water-dispersible.

10. In the method of coating a traveling web of paper, paperboard, andthe like which involves steps of (a) applying to the traveling web afirst layer of coating comprising an aqueous dispersion of mineralpigment having an average particle size up to about 0.8 micron andfilmforming binder, (b) substantially setting the first coating layer,(c) applying over the first coating layer a second layer of coatingcomprising an aqueous dispersion of mineral pigment having an averageparticle size up to about 0.8 micron and film-forming binder, (cl)smoothing the second layer of coating with a doctor blade, and (e)substantially drying the resultant coated web, the improvement for thereduction of scratching in the second coating layer which comprisesapplying as the second coating layer a coating wherein the mineralpigment comprises at least about 20% of a coarse pigment material whichhas an average particle of at least about 1.6 microns.

11. The method of claim 10 in which the coarse pigment material isselected from the group consisting of ground calcium carbonate andsilica.

12. The method of claim 10 wherein the binders in the coatings employedin steps (a) and (c) are Water-dispersible.

References (Iited UNITED STATES PATENTS 1,913,329 6/1933 Bradner1l7--156 X 2,395,992 3/1946 Clark 117156 X 2,498,005 2/1950 Rafton106-306 X 2,666,718 1/1954 Davis 117--156 X 2,668,749 2/1954 Mettan106306 X 2,676,118 4/1954 Thomas 117-83 X 2,685,571 8/1954 Stinchfieldet a1. 1l7-156 X 2,949,382 8/1960 Dickerman et a1. 11783 X 3,015,5751/1962 McKnight et al. 117-156 X 3,132,042 5/ 1964 Weber. 3,136,6526/1964 Bicknell. 3,159,487 12/1964 Krieger et a1. 3,163,535 12/1964Straw. 3,197,322 7/1965 Maskal et al 106-306 3,272,772 9/1966 Russell106306 X 3,288,632 11/1966 Rush et al. 11768 WILLIAM D. MARTIN, PrimaryExaminer.

M. R. LUSIGNAN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,362,845 January 9, 1968 Maurice M. Brundige It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 1, line 22, after "Ser." insert No. column 5, line 2, for "or"read of line 65, after "scratching" insert in line 75, for "has" readhad column 7, line 40 for "micro" read micron column 8 line 40 after"particle" insert size Signed and sealed this 11th day of February 1969.

(SEAL) Attest:

Edward M. Fletcher, Ir. EDWARD J. BRENNER Attesting Officer Commissionerof Patents

